This invention relates to calibration systems and attenuators, and particularly to circuits and methods for calibrating step attenuator inputs of test and measurement instruments and outputs of signal generators.
In the field of test and measurement it is often desirable to provide an instrument, such as an oscilloscope or spectrum analyzer, with the capability of performing self-calibration. Modern computer processor based instrumentation permits this to be done using computer software embedded in the instrument. In such instruments an analog input signal is converted to digital form so that signal measurement and analysis can be carried out by a computer processor in the instrument. A common method for performing self-calibration is to provide a known stimulus, such as a known voltage, current or frequency, to the instrument and program the instrument to calibrate itself to provide a desired response to that known stimulus.
Ordinarily, when a calibration stimulus signal and a signal to be measured are applied simultaneously to an instrument, the instrument response is misleading. Switching the instrument input between the user input, to which the signal to be measured is applied, and the calibration input, to which the calibration stimulus is applied, is a common solution to the problem. Disadvantages to this solution are that the switch introduces a source of signal degradation at high frequencies, characterized by reduced bandwidth and increased rise time, and added expense.
In addition, it is common to utilize a step attenuator at the signal input of such a test or measurement instrument or at the output of a test signal source such as a signal generator. Prior step attenuators ordinarily comprise a number of attenuator pads in series and corresponding switches for switching the pads into or out of the signal path. Input signals pass through each switch, regardless of whether its corresponding pad is switched in or switched out. A disadvantage of such attenuators is that each switch degrades the input signal. Degradation may be kept to an acceptable level by using high performance switches, but such switches are intolerably large and comparatively expensive.
Moreover, in calibrating an instrument having a step attenuator, the instrument should be calibrated for each of the attenuation steps. Where a step attenuator is used and the attenuator input is switched between the user input and the calibration input, the problem of signal degradation due to the introduction of switches is compounded.
Therefore, it can be seen that there is a need for a calibration system that permits an instrument to respond either to a signal to be measured or to a calibration signal without introducing the degradation ordinarily associated with the use of a switch and without producing an unreliable measurement. There is also a need for a method for employing a step attenuator in an instrument without introducing signal degradation.